A photoresistor is an electronic component whose resistance is inversely proportional to the intensity of incident light. A photodetector is a device used to detect electromagnetic radiation. Photoresistors have been known for many decades. The following is a non-exhaustive list of US Patents describing one or more aspects of photoresistors (all of which are incorporated herein by reference):                1. U.S. Pat. No. 3,208,022        2. U.S. Pat. No. 6,133,991        3. U.S. Pat. No. 6,534,808        4. U.S. Pat. No. 3,187,414        5. U.S. Pat. No. 3,211,911        6. U.S. Pat. No. 2,916,678        7. U.S. Pat. No. 3,725,660        8. U.S. Pat. No. 5,309,132        9. U.S. Pat. No. 8,912,478        10. U.S. Pat. No. 3,436,613        11. U.S. Pat. No. 3,978,509        12. U.S. Pat. No. 9,130,104        
Silicon photoresistors were first built using bulk high resistivity silicon with contacts applied across the bulk body. However, those photoresistors were either rather expensive for mass production or did not show high performance.
With the introduction of photoresistors based on cadmium sulphide (CdS) and cadmium selenide (CdSe), the industry offered the market high performance, low cost devices which are currently widely used in various field of technology. Among the advantages of CdS/CdSe-based photoresistors are wide range of dark resistance, high sensitivity to light, and ease of manufacturing.
More recently, the Restriction of Hazardous Substances (RoHS) also known as Directive 2002/95/EC, was originated in the European Union and restricts the use of six hazardous materials found in electrical and electronic products. Faced with changes in regulation requiring RoHS compliant components, the industry is looking for alternatives which may replace photoresistors that contain prohibited substances like Cadmium. Besides, CdS/CdSe-based photoresistors have several drawbacks. For example, their performance severely deteriorates at high temperatures. In addition, the large performance variance and lag (latency) in response to rapidly changing light conditions limit the use of CdS/CdSe-based photoresistors.